the difference between gt3 and gt2 is that gt2 is actually gte, which means cars allowed to be designed as racecars which could use a roadmodel car as base of which x number has to be sold (thought it was 25, but not sure).

gt3 cars are spec cars that roll off the factory line as is and can be bought by any1.

gt2 has more power, but worse brakes and less aero, which results in gt3 cars being faster through the corner, but slower on the straights.

as for which car is better in their class, also depends on your personal driving style, but as for the gt3 class, the z4 is the easiest to drive, its very forgiving, so thats why a lot of people pick it.

other than that, i think the BoP between gt3 cars right now is fairly even.

that can not be said for gt2 and its honestly a shitshow.